In one such known clamping device (cf. company brochure of the company HILMA-ROMHELD GMBH, D-57260 Hilchenbach (Germany), "Technische Daten/Zubehor, Doppelspannsystem DS 125" [Technical data/accessories double clamping system DS 125], 4.3660, page 1, issue 3/96) the abutment part is arranged to slide in the body by the operating end of the one slide. A clamping screw is provided in the abutment part, running perpendicular to the base part of the body, with which the abutment part can be clamped fast relative to the body, when the so-called "THIRD HAND function" described in further detail below is desired. Two helical springs are provided as the spring arrangement, and a each supported at one end on the outer side of the abutment part facing away from the slide and at the other end on the head of a retaining screw, which passes through the abutment part and is screwed into the slide. The abutment part is normally pressed on to the slide by this spring arrangement and is displaced together with the slide. Two workpieces can be clamped at the same time by actuation of a single screw spindle in the known clamping device and they can even have different dimensions. One of the workpieces is clamped between the jaw of the first slide at the operating end and the central jaw, the other workpiece between the jaw of the second slide and the central jaw. Before the clamping it is mostly necessary to align each of the workpieces accurately relative to the body or the jaws. With many workpieces it is necessary to hold the workpiece in the aligned position long enough, until the jaws are pressed with sufficient force on to the workpiece. If however there are two workpieces which have to be held in the aligned position, two hands are needed for this and there is then no hand free for actuating the screw spindle. This problem also always occurs when the body is arranged vertically on a machine table. The so-called "THIRD HAND function" is provided in order that one can firstly align one of the workpieces in such a clamping device with only one screw spindle and then clamp it enough for its to be retained in the aligned position between the jaws. In this case, by first turning the screw spindle the spacing required for the second workpiece between the second jaw and the central jaw is created and this workpiece is then placed between these two jaws. By drawing the slide and jaw unit on the operating end towards the operating end, the second jaw is brought into abutment with the second workpiece and the latter with the central jaw, without play. While maintaining this contact free from play the distance between the operating end first jaw and the central jaw is so adjusted by further rotation of the screw spindle that the first workpiece can just be fitted between these two jaws. The abutment part is clamped fast relative to the body in this position by actuating the clamping screw. The screw spindle is now actuated, whereby the second workpiece is gripped between the second jaw and the central jaw and is held in the aligned position, but is not clamped fast. This gripping replaces the third hand and is therefore called the THIRD HAND function. The gripping of the second workpiece has been made possible in that the first slide is initially held immovable relative to the body by the clamped abutment part and the spring arrangement. After the second workpiece has been gripped in the described manner, one hand is free again and can be used to fit the first workpiece between the jaw of the first slide and the central jaw. On further rotation of the screw spindle the first slide is now displaced against the force of the spring arrangement towards the first workpiece and the first workpiece is gripped between the jaw of the first slide and the central jaw. On further rotation of the screw spindle the two workpieces are finally clamped. When the clamping device is to be released again after completion of machining of the workpieces, the jaws of the first workpiece come free first, while the second workpiece remains gripped in the described manner with the THIRD HAND function turned on. This has the advantage with a vertically standing body that the second workpiece does not fall out of the clamping device when the first workpiece is taken out by one hand and the screw spindle is actuated with the other hand.
Since the abutment part in the known clamping device is arranged alongside the first slide, swarf can fall during machining between the first slide and the abutment part. If this is not removed carefully before unclamping the THIRD HAND function no longer works properly. Moreover either a greater total length of construction of the clamping device or a smaller opening stroke of the jaws results from the arrangement of the abutment part beside the first slide.
In a similar known clamping device (U.S. Pat. No. 5,098,073) the abutment part is arranged outside on the face of the operating end of the body. It surrounds a sleeve concentric with the spindle and axially immovable relative thereto and can be clamped fast thereon in different axial positions. The abutment part also engages slightly in a recess provided in the face of the operating end. The abutment part comprises a lug underneath the spindle, which also engages in the end recess. An annular shoulder is provided in a bore of the lug and a spring arrangement engages on each of the two sides thereof A retaining screw, on which one of the spring arrangements bears, is passed through both spring arrangements and screwed into the base part of the body. The spindle is held axially immovable relative to the body by these two spring arrangements, until one of the movable jaws bears on the first workpiece. This clamping device also has the disadvantage that the abutment part and the sleeve also project for their full length beyond the end face of the operating end and thus substantially increase the overall length of construction of the clamping device. Swarf can also fall between the abutment part and the recess and affect the function.
Another known clamping device (U.S. Pat. No. 4,934,674) has the same disadvantage, being very similar to the previously described clamping device. In this clamping device also the abutment part surrounding the spindle is disposed outside the end of the body, so that the overall length of construction of the clamping device is increased.